This invention relates to a bone replacement material on the basis of calcium phosphate apatites having carbonate and alkali portions, all ions being definedly integrated in the crystal lattice, for the filling of bone defects, for building up bones, for bone-contact layers on replacements of bones and roots of teeth and for independent smaller bone replacement parts and implants, as well as the production and processing thereof.
Hereinafter, the material according to this invention is referred to as MeCO.sub.3 apatite for reasons of simplification; Me stands for metal ions, CO.sub.3 stands for carbonate ions, apatite stands for calcium phosphate apatite.
Owing to the rapid development of surgery, it is nowadays possible to carry out operations to bones and joints which were still inconceivable a little while ago. For example, it is now possible to carry out surgical removal of cysts, foci of suppuration in bone and malignant tumors from bones. This results in defects in the bone, which need to be filled since normal bone repair processes are no longer able to compensate them. Some defects of this type may have a volume of up to 600 cm.sup.3 which has to be filled again. Cavities are also produced during the treatment of teeth, which have to be filled again.
For filling cavities of this type use is made of bone replacement materials in liquid, pasty or solid form as granules or articles for implantation. If the cavities which are to be filled are not too large then the purpose of the bone replacement materials is to temporarily fill the cavities in the bone and to allow the body itself to compensate, in the course of time, the defect with living bone material. This may entail there being either growth around the replacement material, which stays in place without irritation, or slow breakdown and replacement thereof by living bone.
It is necessary to use a material which is compatible with bone for filling larger cavities with bone replacement material. The materials of this type which are used are endogenous or exogenous fragments of bone or hydroxyapatite granules. Only very limited amounts of endogenous bone material are available, and additional surgical operations are necessary to obtain it. It is necessary to remove all antigens from exogenous, for example animal, bone materials in order to avoid rejection reactions, but this is only partially successful in practice.
When hydroxyapatite is used there is primarily irritation of the surrounding bone material. Thus, there is a need for a material which can be used in liquid, pasty or solid form for filling cavities in bones, that is to say for filling bone defects and which does not cause any primary irritations.
Bone implants are frequently used in surgical operations. Bone implants are items which are implanted in the bones of the body of a recipient and permanently replace parts of the skeleton or roots of teeth. The outer layer of the bone implant, which comes into contact with the living substrate bone, is termed the bone-contact layer. At the present time, metals, such as, for example, special steels, noble metals, titanium, ceramic materials, such as, for example, alumina, glass-ceramics, hydroxy-apatite ceramics and synthetic materials are used as bone implants and as bone-contact layers.
These substances are classified as biocompatible and bioactive according to the tissue compatibility. Biocompatible substances are tolerated by the body in the long term without rejection. Bioactive substances become rigidly incorporated like endogenous tissue, the tissue compatibility being determined by the chemical composition, the crystalline structure, the surface structure and the mechanical properties.
The metals and some ceramic materials, such as, for example, alumina ceramics, are biocompatible. Ensheathing by connective tissue always takes place in the body. This connective tissue layer allows the implant to be held relatively rigidly, but does not allow frictional connection to the mineral framework of the substrate bone.
Because of the absence of primary integration into the substrate bone, a biocompatible implant of this type can be exposed to only slight mechanical stress since otherwise it is held increasingly poorly, and this is associated with pain and, finally, the loss of the implant. This is found, for example, with hipjoint prostheses, which are always subject to great stress and for which nowadays more than one quarter of the operations are carried out because of loosening of an implant which had previously been inserted.
Thus, additional undercutting such as, for example, a screw thread is necessary for permanent mechanical anchoring of biocompatible implants in bone. With all metallic implants it is still an unanswered question of whether they release toxic metal ions into the surroundings and thus may have adverse effects in the long term.
Even when bone cement is used, despite the initially better mechanical connection to the substrate bone, a loosening which has been described takes place, with some delay.
In the case of bioactive materials, after some time the bone material grows directly thereon. Among the known materials, the best properties in this respect are shown by hydroxyapatite which, after a period, which lasts only a few weeks, of mild signs of irritation, which can be detected under the microscope by giant cells around the implant, is integrated into the substrate bone without an interlayer.
As a rule, bioactive materials are difficult to work and less mechanically stable than the biocompatible metals or ceramics.
Thus, there has been a change to the use of combined implants comprising biocompatible cores such as, for example, titanium, special steel and alumina, and bioactive surface coatings (compare German Patent Specification No. 2,840,064). A combined implant construction of this type may have considerable advantages since, in this case, high mechanical stability of complex-shaped implants is combined with rapid and rigid connection to the substrate bone. Again, signs of irritation appear after the insertion of implants of this type with the bioactive coatings hitherto known, but these signs subside after some time. Thus, there is a great need for a material which can be used for the production of implants or for the production of bone-contact layers for implants, and which has the necessary bioactive and biomechanical properties and enables osteointegration without irritation